One of the oldest legal principles, expressed by English jurist William Blackstone in the 18th century, is that it is better for 10 guilty people to escape than one who is innocent to suffer. The courts can only convict suspects if they are sure they are guilty. Suspicion is not enough. This is why guilt must be proved beyond reasonable doubt. The importance of the high threshold is clear as Hong Kong’s courts try around 2,500 people for alleged crimes arising from the civil unrest of 2019. These cases often involve chaotic events such as mass protests and clashes with police. The gathering of evidence is challenging. But the prosecution must ensure they have a sufficiently strong case if they are to proceed to court. A decision by the Court of Appeal on Thursday will make prosecutions in such cases easier to bring and convictions more likely. The court held the principle of “joint enterprise” applies to charges of riot and illegal assembly under the Public Order Ordinance. Those involved in a joint enterprise are held responsible for a crime even if they did not actually commit it. They might have planned, funded, assisted or encouraged the offence. A classic example is the person who hires a contract killer. They do not need to be present at the scene of the crime. The judges described joint enterprise as a tool developed by the courts to overcome evidential uncertainties in trying to prove a suspect is an accessory to a crime. The court’s ruling was sought by the Department of Justice to clarify the law in a case in which three alleged rioters had been acquitted. There was no attempt to overturn their not guilty verdicts. Prosecutors must be careful not to apply joint enterprise too broadly in protest cases. Many of those at protests where trouble flared were not directly involved. What conduct will be required to turn a bystander into a rioter? Wearing black clothes? Running away when the police move in? Liking a social media post promoting a protest that later turns violent? Court sides with justice department in allowing ‘joint enterprise’ riot prosecutions The court dismissed concerns use of the principle could lead to innocent onlookers being convicted. Each case will depend on the circumstances and the evidence. The judges said a peaceful protester should leave the scene as soon as possible if the demonstration becomes unlawful. If there is a good reason for staying, the suspect will not be liable. It will be for the courts to decide whether defendants have crossed the line and become involved in a breach of the peace. The conviction rate for contested riot cases arising from the protests has been low. Six defendants have been found guilty after trial, 25 have been acquitted. Seventeen pleaded guilty. Why have more than 80 per cent of those pleading not guilty been acquitted? Claims of judicial bias can be swiftly dismissed. The Department of Justice might argue courts have misunderstood the law. It is appealing against 18 of the acquittals. The courts have taken the view that the prosecution case was not strong enough. The evidence in these trials has been mostly circumstantial. Courts have been asked to infer defendants were involved in rioting because, for example, they were nearby, wearing black, or in possession of protective gear. The judges have understandably been reluctant to convict on that basis. Whether the joint enterprise ruling changes this remains to be seen. The prosecution has also had problems with unreliable evidence and identifying the offender. The prosecution should not cast its net too wide. The Department of Justice’s Prosecution Policy states there must be sufficient evidence to demonstrate a reasonable prospect of a conviction if a case is to be taken to court. “The public interest is not served by proceeding with cases that do not satisfy that test,” it adds. Strict adherence to this principle is needed to ensure that only the guilty are convicted and the innocent do not suffer.